SUMMARY

Hovering flight is one of the most energetically demanding forms of animal
locomotion. Despite the cost, hummingbirds regularly hover at high elevations,
where flight is doubly challenging because of reduced air density and oxygen
availability. We performed three laboratory experiments to examine how air
density and oxygen partial pressure influence wingbeat kinematics. In the
first study, we experimentally lowered air density but maintained constant
oxygen partial pressure. Under these hypodense but normoxic conditions,
hummingbirds increased stroke amplitude substantially and increased wingbeat
frequency slightly. In the second experiment, we maintained constant air
density but decreased oxygen partial pressure. Under these normodense but
hypoxic conditions, hummingbirds did not alter stroke amplitude but instead
reduced wingbeat frequency until they could no longer generate enough vertical
force to offset body weight. In a final combined experiment, we decreased air
density but increased oxygen availability, and found that the wingbeat
kinematics were unaffected by supplemental oxygen.

We also studied hovering and maximally loaded flight performance for 43
hummingbird species distributed along a natural elevational gradient in Peru.
During free hovering flight, hummingbirds showed increased stroke amplitude
interspecifically at higher elevations, mirroring the intra-individual
responses in our first laboratory experiment. During loaded flight,
hummingbirds increased both wingbeat frequency and wing stroke amplitude by
19% relative to free-flight values at any given elevation. We conclude that
modulation of wing stroke amplitude is a major compensatory mechanism for
flight in hypodense or hypobaric environments. By contrast, increases in
wingbeat frequency impose substantial metabolic demands, are only elicited
transiently and anaerobically, and cannot be used to generate additional
sustained lift at high elevations.

Introduction

Animal flight at high elevations presents a double physiological challenge:
reduced air density demands higher lift and mechanical power output, whereas
reduced oxygen availability can constrain metabolic power input. Two of the
most important mechanisms for generating higher lift in hovering flight are
(1) to increase the horizontal area swept out by the wings and (2) to increase
the wingbeat frequency (
Ellington,
1984). However, muscular activity and correlated wingbeat
kinematics can also be adversely affected by reduced oxygen availability. How
do animals satisfy the conflicting requirements of flight at high elevations?
The purpose of this study was to determine how hummingbirds modulate wingbeat
kinematics in both reduced air density and reduced oxygen partial pressures.
We present results from laboratory and field experiments that measure both
sustained and burst kinematic performances of hovering hummingbirds under
different conditions of hypodense, hypoxic and hypobaric air.

Hummingbird flight has been previously examined in a suite of experimental
studies that evaluated hovering in normoxic but hypodense air
(
Chai and Dudley, 1995), in
hypoxic and hypodense air (
Chai and Dudley,
1996), in hyperoxic and hypodense air
(
Chai et al., 1996), and during
maximal load-lifting (
Chai et al.,
1997;
Chai and Millard,
1997). The principal conclusion from these experiments is that
when challenged by low-density air, hummingbirds exhibit considerable
modulation in stroke amplitude, ultimately reaching a constraint on flight
performance in low-density air when the stroke amplitude reaches a geometric
limit near 180° (
Chai and Dudley,
1995). In contrast, hummingbirds increase wingbeat frequency only
marginally in hypodense air, and then only if the air is normoxic. Here, we
present kinematic data for these and other similar experiments that isolate
the effects of air density and oxygen partial pressure, with specific
reference to elevational gradients in the field. Specifically, we determined
wingbeat kinematics for hummingbirds hovering in two experimental conditions
that mimic effects of increasing altitude: (1) decreased air density, attained
by replacing normal air with normoxic heliox, and (2) hypoxia, attained by
replacing normal air with pure nitrogen. In addition, we performed a third
experiment to test metabolic performance with reference to elevation, by
filming birds in hypodense hyperoxia, which we accomplished by replacing
normal air with hyperoxic heliox.

The hummingbird family is species-rich, comprising over 320 species found
exclusively in the New World (
Schuchmann,
1999); however, comparative data on hummingbird wingbeat
kinematics are confined to data sets on body mass and wingbeat frequency
(
Greenewalt, 1975). Curiously,
the allometry of hummingbird wingbeat frequencies follows a different scaling
relationship relative to that of insects. In a non-phylogenetic analysis,
insect wingbeat frequencies scale with mass–0.24, whereas
hummingbird wingbeat frequency scales significantly differently, with
mass–0.61 (
Dudley,
2000). Thus, relative to insects, hummingbirds wingbeat
frequencies decline more rapidly with increased body mass.

Hummingbirds occur at almost all elevations in the Americas where there are
flowering plants (
Schuchmann,
1999), and some Andean hummingbirds subsist on flowers at
elevations as high as 5000 m (
Carpenter,
1976). Thus, the Andes provide a natural context for studying
hummingbird flight across elevational gradients. We filmed hovering and
load-lifting for an assemblage of hummingbirds over a ∼4000 m elevational
range in southeast Peru. The kinematics of both types of flight were examined
with reference to both elevation and body mass.

Materials and methods

Hummingbird flight experiments were performed using chambers sufficiently
small to allow for bird restraint and filming but large enough to prevent
boundary effects. Specifications and sizes for the flight chambers in each of
the experiments are given below. In all experiments, a video camera (Sony
Video 8 CCD-TR44) was used to record wingbeat kinematics at 60 frames
s–1 with a high-speed shutter of 1/4000 s. The camera filmed
a mirror positioned at 45° above the flight chamber to obtain horizontal
wing projections and flight kinematics. Horizontal projection of wing motions
yielded accurate measurements of wing positional angles because the stroke
plane angle of hummingbird wings is close to zero (e.g.
Chai and Dudley, 1996;
Stolpe and Zimmer, 1939).
Because the camera filmed at 60 frames s–1 and hummingbird
wingbeat frequencies varied from 14 to 75 Hz, it was not possible to film
multiple frames per wing stroke for most of the hummingbirds. Instead, we
analyzed kinematics over many wing stroke cycles corresponding to periods of 2
s for all experiments, except for load-lifting (see below).

In frame-by-frame analysis of video films, we measured two features of
wingbeat motion in each of the experiments: wingbeat frequency and wing stroke
amplitude (
Chai and Dudley,
1996). Wingbeat frequency (N) is the number of complete
wingbeats per second (Hz), and is determined from the interaction frequency
between the wingbeat frequency and the filming rate (60 frames
s–1) of the video camera. For a hummingbird hovering with a
wingbeat frequency of 60 Hz, the wing would appear stationary. When the
wingbeat frequency exceeds 60 Hz, the wing appears to move forward in
consecutive frames, whereas wingbeat frequencies less than 60 Hz result in a
film sequence in which the wing lags behind. Thus an average wingbeat
frequency can be calculated by counting the number of apparent wingbeat cycles
completed over the course of 1 s, and then by adding this number to 60 if the
wing appears to move forward, or subtracting it from 60 if the wing appears to
move backwards.

The stroke amplitude (Φ) is the angular extent of wing motion within
the stroke plane, and was determined from the subset of frames within the
sequence used to calculate wingbeat frequency. At the end of either
halfstroke, the wings appear as stationary thin lines when viewed from above.
For these moments, wings were oriented vertically and thus were either in the
middle of pronation or supination. The angular extents of wing motion were
measured using a protractor, with the angle between the midpoints of pronation
and supination termed the wing stroke amplitude.

Decreased air density trials

Density reduction experiments were performed at two elevations in the
Colorado Rocky Mountains, USA. The low-elevation site was located in Cheyenne
Canyon Park outside Colorado Springs, CO (1875 m), and was characterized by an
average air density of 0.987 kg m–3 and an average oxygen
partial pressure of 128.3 mmHg (1 mmHg=133.3 Pa). The high-elevation site was
the Rocky Mountain Biological Laboratory (RMBL) in Gothic, CO (2900 m),
characterized by an air density of 0.862 kg m–3 and an oxygen
partial pressure of 112.1 mmHg. These elevations are close to the lower and
upper elevation limits, respectively, of co-occurrence for both rufus
Selasphorus rufus and broad-tailed S. platycercus
hummingbirds, during the breeding season of the latter.

Hummingbirds at both sites were captured in mist nets or feeder traps, and
were then immediately transported to a field laboratory for flight trials.
Density-reduction trials were performed on a total of 24 individuals
comprising 12 individuals at each site: four male S. platycercus
Swainson, four male S. rufus Gmelin and four female S.
rufus. Females of S. platycercus were not used because the study
occurred during their nesting season. Furthermore, long-term studies of
hummingbird population dynamics were ongoing at the RMBL, and thus it was not
permissible to hold breeding females for sufficient time to complete all
flight experiments.

Birds were placed individually in an airtight Perspex cylinder (0.5 m
diameter; 1 m high) within which normodense air at the local ambient pressure
was gradually replaced with normoxic heliox (ρ=0.41 kg
m–3 at sea level), thus maintaining constant oxygen partial
pressure while gradually lowering air density. Birds rested on a retractable
perch that was retracted every few minutes, thus forcing the birds to hover.
Trials progressed until birds could no longer sustain hovering flight. At this
point, the chamber was flooded with normodense air. Air temperature and
humidity were measured directly within the flight chamber; local barometric
pressure was obtained from climatic data collected at each site. These data,
together with a directly measured change in acoustic frequency of a resonant
whistle located within the chamber, enabled calculation of air density
following heliox infusion (
Dudley,
1995).

Data were analyzed using a repeated-measures analysis of variance (ANOVA)
with air density as the repeated independent measure. The species/gender class
of the hummingbird and the starting elevation of the experiment were the other
independent variables. Dependent variables in two separate analyses were
wingbeat frequency and stroke amplitude.

Hypoxia trials

After flight trials in heliox and following a period of feeding, rest and
recovery, the same individual hummingbirds served as subjects for flight
trials in hypoxia. Protocols and analysis were identical to previously
described heliox manipulations except that pure nitrogen was infused into the
flight chamber and gradually replaced both the nitrogen and oxygen of
unmanipulated air. Because the density of nitrogen (ρ=1.165 kg
m–3 at sea level) is very similar to that of normal air, this
manipulation permitted a reduction in oxygen concentration at a near-constant
air density. In addition to measurements of humidity and temperature, we also
recorded instantaneous oxygen concentration of the mixture within the flight
chamber using an Oxygen Monitor (GC Industries GC 501, Poulsbo, WA, USA).

Hyperoxia trials

We studied flight performance in hypodense hyperoxic air in a series of
experiments between 1995 and 1997
(
Altshuler et al., 2001;
Chai et al., 1996). Here we
further analyze for comparative purposes the kinematic data from those
hyperoxia experiments. Our previous articles contain a complete description of
the methods and only a brief description will be given here.

All experiments were conducted with captive ruby-throated hummingbirds
Archilochus colubris L. Hovering flight was studied in a large
plexiglas chamber (90 cm×90 cm×90 cm) in 1995 and 1996, and in a
smaller chamber (60 cm×60 cm×60 cm) in 1997. In the primary
experiment, normal air was replaced with hyperoxic heliox (35% oxygen/65%
helium; ρ=0.57 kg m-3 at sea level). For comparison, the same
birds were also tested in normoxic heliox of similar or equivalent air density
(see
Altshuler et al., 2001).
Effects of gender, oxygen concentration and air density on wingbeat kinematics
were tested using repeated-measures ANOVA, with air density as the repeated
independent measure.

Free hovering of Andean hummingbirds

Our comparative studies on hummingbird flight kinematics were carried out
between June 1997 and August 2000. We visited 11 field sites in the
Departments of Cusco and Madre de Dios in southeastern Peru that spanned
elevations from 400 m to 4300 m. Hummingbirds were captured in mist nets and
were then brought to a field laboratory for measurements and flight trials,
after which they were released. Body mass (m) was determined to
within 0.001 g using an Acculab Digital Scale (Model #PP-2060D, Edgewood, NY,
USA) or to within 0.1 g using a hanging spring balance (Avinet, Dryden, NY,
USA).

Free-flight trials began by releasing a hummingbird into a nylon-mesh
flight chamber (0.9 m high×0.45 m×0.45 m) with a Perspex top. Most
birds initially tried to fly through the Perspex, but quickly learned that it
was impassable. Thereafter, the birds hovered in the chamber for several
minutes before perching on the walls of the chamber. We filmed this hovering
flight and acquired kinematic variables from the video films using the methods
described above. For sexually dimorphic species, kinematic data were pooled by
gender and averaged. Species averages were then calculated as the average of
values for males and females. In species without easily distinguishable sexes,
only one average among individuals was calculated per species.

This study involves the comparative analysis of interspecific data. It is
now widely appreciated that such analyses require an explicit phylogenetic
framework. Felsenstein (
1985)
showed that data points representing species values should not be treated as
independent observations because of the potential confounding effects of
phylogenetic relatedness. We incorporated such phylogenetic influences by
using standardized independent contrasts
(
Felsenstein, 1985) as
calculated by the program CAIC (
Purvis and
Rambaut, 1995). Species data were log-transformed prior to
computing contrasts to remove heterogeneity of variance and correlation
between node values and independent contrasts
(
Garland et al., 1992). The
phylogeny contains 73 hummingbird taxa and was generated using Bayesian
phylogenetic analysis (
Huelsenbeck and
Ronquist, 2001). Two nuclear genes (AK1, Beta-fibrinogen intron 7)
and one mitochondrial gene (ND2) were sequenced and analyzed using a general
time-reversible (GTR) plus site-specific gamma model of evolution (J. A.
McGuire and D. L. Altshuler, unpublished data). Independent contrasts were
calculated by setting all branch lengths to one, which gave equal weight to
each contrast and provided the best method of standardization for regression
of these data (
Garland et al.,
1992). Data were analyzed using multiple regressions of
independent contrasts with two independent variables: body mass and elevation.
All regressions of independent contrasts were constrained to go through the
origin (
Garland et al.,
1992).

Maximum load lifting trials with Andean hummingbirds

In conjunction with free flight trials, Andean hummingbirds were also
tested for maximum load-lifting performance. Complete protocols for asymptotic
load-lifting in hummingbirds are available elsewhere
(
Chai et al., 1997;
Chai and Millard, 1997), and
only a brief account will be given here. A rubber harness connected to a
thread with color-coded beads was placed over the head of each hummingbird.
Hummingbirds were released on the floor of the nylon-mesh flight chamber (0.9
m high×0.45 m×0.45 m). Because the natural escape response of a
hummingbird is to fly directly upwards, we were able to obtain films of
vertical ascent. The typical behavior of a hummingbird during load-lifting is
to fly as high as possible, and then to hover briefly (∼0.5 s) before
descending laterally towards the chamber wall. In addition to a video camera
positioned above the flight chamber that filmed wingbeat kinematics, a second,
synchronized camera (Video 8XR CCD-TRV16; Sony) filmed the floor of the
chamber to determine simultaneously the colors of the remaining beads, and
thus by subtraction, the total weight lifted by the bird. After hummingbirds
had made multiple ascending flights and started to tire, the weight chain was
removed and the trial was ended.

From video films, the maximum weight lifted by each bird was determined and
three flight sequences exhibiting maximum lifting performance were analysed.
Thus, kinematic data were averaged over three bouts of maximum lifting for
1–2 s of total analysed flying time. Interspecies comparisons were made
using phylogenetic controls, as described above.

Results

Decreased air density trials

Morphological and kinematics parameters of the Selasphorus
hummingbirds at the two study sites are given in
Table 1. Wingbeat frequency
increased as air density decreased (F2,34=3.793,
P<0.05), although the differences were slight
(
Fig. 1A). The different
classes of hummingbirds differed in wingbeat frequency
(F2,17=119.395, P<0.0001). Short-winged S.
rufus males exhibited higher wingbeat frequencies than did S.
rufus females with intermediate wing lengths, which in turn had higher
frequency than long-winged S. platycercus males.

Hovering kinematics in hypodense air. As ambient air was replaced with
normoxic heliox, air density decreased but the partial pressure of oxygen
remained constant. The data depicted here were from the experiments performed
at 1875 m, although the same trends were evident at 2900 m. (A) Wingbeat
frequency increased slightly with decreasing air density. (B) Stroke amplitude
increased substantially with decreasing density to a limit near 180°.
Values are means ± s.e.m.

As air density was decreased, Selasphorus hummingbirds also
increased stroke amplitude (
Fig.
1B; F2,34=51.630, P<0.0001) up to
approximately 180°, at which point the birds were unable to hover and
exhibited aerodynamic failure (sensuChai and Dudley, 1995). The
average stroke amplitudes at failure for S. playcercus males, S.
rufus males and S. rufus females were 182°, 182° and
177°, respectively. Values of stroke amplitude greater than 180° can
be reached because the wings are separated by the small distance of the body
width. The hummingbird species/gender classes differed significantly in their
stroke amplitudes across air densities (F2,17=11.677,
P<0.001), and a significant interaction with air density was also
detected (F4,34=3.347, P<0.05). As air density
was decreased, males of S. rufus increased their stroke amplitude
less than did males of S. platycerus, because the stroke amplitude of
S. rufus males was higher than that of other hummingbirds at both
elevations in the study.

Hypoxia trials

As the partial pressure of oxygen was decreased, hummingbirds decreased
wingbeat frequency (F2,26=10.853, P<0.001),
although the overall differences were slight
(
Fig. 2A). Hummingbirds ceased
hovering when ambient air reached a minimum oxygen concentration (aerobic
failure), and then gradually descended to the bottom of the chamber. The
species/gender classes also differed in wingbeat frequency
(F2,13=295.164, P<0.0001), showing the same
patterns with wing size as for flight in low-density air. The average decrease
in wingbeat frequency within a site and across partial pressures of oxygen
from the start of the experiment to the point of aerobic failure for S.
playcercus males, S. rufus males and S.rufus
females was 1.1 Hz, 2.0 Hz and 2.4 Hz, respectively. In addition to the
comparisons within sites, wingbeat frequencies also differed between sites.
Overall, wingbeat frequencies were higher at 2900 m than at 1875 m
(F1,13=18.962, P<0.001), even though less
oxygen was available at the high elevation site. Combining the results of the
air density and the hypoxia experiments indicates that these differences in
wingbeat frequency between elevations were caused by differences in air
density rather than oxygen partial pressure. Furthermore, S.
platycercus males did not significantly increase wingbeat frequency at
high elevations, resulting in a significant species/gender × elevation
effect for wingbeat frequency (F2,13=8.464,
P<0.005). These hummingbirds also have the longest wings of the
hummingbirds sampled, underscoring the fact that lower density required higher
wingbeat frequency, with longer-winged S. platycercus males being
less sensitive to such changes.

Hovering kinematics in hypoxic air. As ambient air was replaced with pure
nitrogen, the partial pressure of oxygen declined but the air density varied
only slightly. Otherwise, conditions were identical to that of the first
experiment described in
Fig. 1.
(A) The wingbeat frequency decreased slightly but significantly as oxygen
partial pressure decreased (see text). (B) Stroke amplitude varied
considerably but exhibited no clear pattern with changing partial pressure of
oxygen. Values are means ± s.e.m. All symbols as in
Fig. 1. 1 mmHg=133.3 Pa.

Unlike wingbeat frequency, stroke amplitude exhibited no specific
relationship with oxygen concentration (P>0.25), although values
were quite erratic (
Fig. 2B).
However, the species/gender classes differed in stroke amplitude
(F2,13=9.874, P<0.005): S. rufus
males had higher stroke amplitudes than did S. rufus females, which
in turn exhibited stroke amplitudes greater than those of S.
platycercus males. This pattern again conforms to differences in wing
length. At higher elevations, all hummingbirds exhibited higher stroke
amplitudes than those hovering at low elevation
(F1,13=45.564, P<0.0001), a pattern clearly
associated with differences in air density between the two sites.

Hyperoxia trials

Kinematics of hovering performance in hyperoxia were compared to those in
normoxia at equivalent air densities (
Fig.
3). Supplemental oxygen had no effect on either wingbeat frequency
or stroke amplitude (all trials characterized P>0.80), however,
indicating that oxygen partial pressure per se was not limiting to
hummingbirds, even when flying at very low air densities near aerodynamic
failure.

Kinematics in hyperoxic air. As ambient air was replaced with hyperoxic
heliox, air density decreased and oxygen concentration increased
simultaneously. During normoxia trials, ambient air was replaced with normoxic
heliox so that air density decreased but oxygen concentration remained at 21%.
Hummingbird kinematics in hyperoxia were equivalent to those in normoxia.
Values are means ± s.e.m. See text for details.
%O2 is the oxygen concentration at each density under
hyperoxia.

These hyperoxia experiments also allowed for further comparison between
sexes and across air densities. With respect to intersexual differences,
Archilochus colubris males have much shorter wings and also much
higher wingbeat frequencies than do females
(F1,14=182.295, P<0.0001). As air density
decreased in hyperoxia, all A. colubris increased both wingbeat
frequency (F4,56=21.61, P<0.0001) and wing
stroke amplitude (F4,56=56.208, P<0.0001).

Free hovering of Andean hummingbirds

Video recordings of free hovering flight were made for 347 individual
hummingbirds from 43 species. Of these species, 38 were represented in the
present phylogeny. Thus, the raw species analyses
(
Fig. 4) consisted of 43 data
points whereas the phylogenetically controlled analyses
(
Fig. 5C,D) are based on 37
(i.e. N–1) contrasts.

Wingbeat kinematics during hovering across a natural elevational gradient
in the Peruvian Andes. Data are species mean for 43 species of hummingbirds.
(A) Wingbeat frequency decreased with increasing body mass. The largest
hummingbird is the giant hummingbird Patagona gigas, which is
substantially larger than all other trochilid taxa and is considered an
outlier. However, the decrease in wingbeat frequency with body mass is found
even if P. gigas is removed from the analysis (inset; all
P<0.001; see text). (B) Stroke amplitude increased with increasing
elevation, mirroring the results of experiment 1
(
Fig. 1). See text for
regression equations.

Wingbeat kinematics during free hovering and load-lifting in Peruvian
hummingbirds. (A,B) Raw species data, (C,D) phylogenetically corrected
independent contrast data. Solid lines, free flight; broken lines,
load-lifting. Wingbeat frequency decreased with increasing body mass during
both load-lifting and free hovering flight. Stroke amplitude increased with
increasing elevation during free flight, but not with elevation during
load-lifting, because all hummingbirds reached a maximum stroke amplitude of
approximately 180° at the point of maximum lifting. See text for
regression equations.

Maximum load lifting trials with Andean hummingbirds

Results of load-lifting experiments, along with the kinematic parameters
for free hovering flight, are plotted in
Fig. 5 as functions of both
body mass and elevation. Both raw species data and phylogenetically controlled
independent contrasts are included.

As in free hovering, wingbeat frequency was unrelated to elevation during
maximum load-lifting (all P>0.15), but was negatively correlated
with body mass for both raw interspecfic data and for the independent
contrasts (
Fig. 5C;
y=–0.43x, P<0.0001). Wingbeat frequency during
load-lifting was considerably higher than during free flight; hummingbirds
increased wingbeat frequencies among species and across elevations by 19%, on
average, relative to free hovering flight
(
Fig. 5A). Thus, hummingbirds
possess the ability to modulate frequency upwards over very short time spans,
but do not use this ability during sustained hovering, as was required during
the density-reduction experiments.

Stroke amplitude increased with elevation during free flight, but was
independent of elevation during load lifting for both raw species data and
phylogenetically controlled contrasts (
Fig.
5B,D; P>0.15 in both cases). Furthermore, stroke
amplitude during maximal load lifting reached a geometric limit between
176° and 201°. Thus hummingbirds were unable to increase stroke
amplitude with increasing elevation because all hummingbirds used the maximum
stroke amplitude near 180° during maximum loading
(
Fig. 5B). On average,
hummingbirds increased stroke amplitude by 19% in loading relative to free
flight. As for free flight, loaded stroke amplitude was independent of body
mass (P>0.2 in both cases).

Discussion

Kinematic mechanisms employed to augment lift production vary with both
oxygen availability and air density. During sustained flight in hypodense air,
hummingbirds increase lift primarily via modulation of wing stroke
amplitude, with relatively constant wingbeat frequency. When generating
transiently high vertical forces, however (probably via anaerobic
pathways;
Chai et al., 1997),
hummingbirds also significantly increase wingbeat frequency. Sustained
increases in wingbeat frequency would require additional and probably limiting
oxygen delivery to the flight muscles, whereas stroke amplitude can be
increased substantially under normoxic and even hypoxic hypobaric conditions.
These patterns were demonstrated both in laboratory experiments and in the
comparative field study of Andean hummingbirds.

Hummingbirds, like other birds, are clearly resistant to low oxygen partial
pressures. In hypoxia, hummingbirds exhibited only slight decreases in
wingbeat frequency and were able to fly at oxygen partial pressures equivalent
to the hypobaric hypoxia of 6000 m or more. Diverse morphological and
physiological adaptations of hummingbirds in particular, and of many birds in
general, enhance oxygen delivery under such conditions
(
Altshuler et al., 2001).

During anaerobic burst performance, hummingbirds increased wingbeat
frequency by 19% on average, which is the same percentage increase as
exhibited in modulation of stroke amplitude. Thus, the kinematic potential to
increase lift via modulation of wingbeat frequency is substantial
but, in conjunction with concurrent increases in stroke amplitude and
metabolic demand, probably reaches a constraint on rates of oxygen delivery at
some stage of the cardiovascular or respiratory system. Because hummingbirds
did not increase their wingbeat frequency when supplied with supplemental
oxygen during a hypodense challenge (
Chai
et al., 1996), diffusion limitations within the pathway for oxygen
are unlikely to pertain. Many birds exhibit multiple exchanges of air in the
lung per inspiration (
Dubach,
1981), although no pulmonary adaptations specific to hummingbirds
have been identified. Convective limitations either in the lungs or the
cardiovascular system (
Bishop,
1999) probably limit oxygen delivery during the maximum hovering
performance of hummingbirds, but the precise nature of such limits remains an
open question.

Although air density and oxygen concentrations were monitored during the
gas infusion trials, we did not measure changes in the partial pressure of
CO2. Disruption of normal CO2 levels can alter
chemoreceptor reflexes in birds, but these are typically detected at
concentrations much higher (2–5%) than ambient (0.03%) (e.g.
Butler and Stephenson, 1988).
If anything, CO2 concentration decreased during gas infusion trials
to potentially half of the ambient concentration. Thus, we cannot exclude the
possibility that hypocapnia adversely affected hummingbird performance.

Although stroke amplitude is a major mechanism for increasing lift
production during sustained hovering, different hummingbird taxa are not equal
in their ability to modulate it. For Selasphorus hummingbirds in
Colorado, the birds with the higher wing disc loading (S. rufus
males) exhibit higher stroke amplitude during free hovering. Thus, they are
unable to increase stroke amplitude as much as hummingbirds with lower wing
disc loadings and stroke amplitudes, and consequently exhibit aerodynamic
failure at higher air densities. Another intra-site comparison is available
for a set of three hummingbird species from southeast Arizona at an elevation
of 1676 m (
Chai and Millard,
1997). Here, the two heavier species had much higher stroke
amplitudes than did the lighter species. Finally, Andean hummingbird species
at higher elevations also exhibited a decreased ability to increase stroke
amplitude. In each case, the ceiling is set by the geometric limit near
180°, beyond which angle the contralateral wings interfere with one
another, either aerodynamically or physically
(
Chai and Dudley, 1995).

Wingbeat frequency is highly correlated with body mass as shown for the
Peruvian hummingbirds, but this is likely to be an indirect correlation
arising from associations with wing inertia. Body mass of flying animals is
itself generally correlated with wing length, which is a strong predictor of
wingbeat frequency (Greenewalt,
1962,
1975). For the Colorado
Selasphorus hummingbirds, each of the species/gender classes is of
similar body mass, but the wingbeat frequencies segregate according to
differences in wing length (Figs
1,
2;
Table 1). The body sizes of
North American hummingbirds, however, are quite similar relative to the wide
range of size variation present in tropical hummingbird communities. For
example, one of the smallest vertebrate endotherms is the Cuban bee
hummingbird Mellisuga hellenae, with individuals as small as 1.8 g.
The largest hummingbird is the giant hummingbird Patagona gigas,
which reaches a body mass of 24 g. The hummingbird assemblage from southeast
Peru studied here includes most of this mass range; the smallest individual
studied was a 2.35 g reddish hermit Phaethornis ruber and the largest
the giant hummingbird. After incorporating phylogenetic relatedness of
species, it was determined that wingbeat frequency scaled as body
mass–0.466 during free hovering, and as body
mass–0.429 during maximal load-lifting for this assemblage.
Although these values cannot be compared with phylogenetically controlled
estimates for insects, it is likely that this decline in wingbeat frequency
with increasing body mass is still much steeper in hummingbirds, for reasons
as yet unclear but likely to relate to the positive allometry of hummingbird
wing area (
Dudley, 2000).

It is now clear that hummingbirds are capable of considerable modulation of
wingbeat frequency and stroke amplitude, although the magnitude of such
responses can vary according to morphological features and elevational
occurrence. Nonetheless, little is known about modulation of detailed wingbeat
kinematics, including such features as angle of attack, torsion along the
wing, wing rotational velocities, and temporal changes in wing area related to
positional changes of the feathers
(
Altshuler and Dudley, 2002).
Although future studies of hummingbird adaptation to the aerodynamic
challenges of hypobaria should consider a wider range of kinematic parameters,
it is nonetheless striking that the behavioral responses to low-density air
exhibited by individual hummingbirds are mirrored in evolutionary time by
hummingbird taxa adapted to flight at varying elevations.

ACKNOWLEDGEMENTS

We thank Peng Chai for deep insights into hummingbird flight performance.
We also thank Peter Baik, Jeff Chen, Jeremy Goldbogen, Holly Hughes and Andrea
Smith for assistance with video analysis. Laboratory and field experiments
were supported by grants from the Earthwatch Institute, the Explorers Club,
the Graduate Program in Zoology at the University of Texas, the Institute for
Latin American Studies at the University of Texas, the National Science
Foundation (IBN 9817138, IBN 992155 and DEB 0108555), Sigma Xi, and the
Graduate School of the University of Texas at Austin.

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Erika Eliason is an Assistant Professor at University of California, Santa Barbara, USA, where she studies ecological and evolutionary physiology. She shares how a childhood spent discovering the natural world in the woods where she grew up ignited her love for science and fieldwork, why conferences should be more family-friendly and the importance of being prepared for the unexpected.

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Meet the team

Are you going to the 2018 APS Comparative Physiology meeting in October? This meeting is supported by JEB and our Reviews Editor Charlotte Rutledge will be there - stop by and say hello! Advance registration is open until 24 September 2018.